This invention relates to coating substrates, particularly glass substrates, by chemical vapor deposition. This invention more particularly relates to a method for preparing gas-reactant vapor mixtures for use in such chemical vapor deposition methods.
Chemical deposition processes such as pyrolytic processes and hydrolytic processes are well known in the art of coating substrates. The physical form of the coating reactants employed in such processes may be liquid, vapor, liquids or solids dispersed in gas mixtures or vaporized or vaporous coating reactants dispersed in gas mixtures. The use of vaporized or vaporous coating reactants is characterized as a chemical vapor deposition process. U.S. Pat. No. 3,852,098 to Bloss et al teaches the vaporization of dispersed solid coating reactants to provide a reactant vapor suitable for reaction to deposit films on substrates, such as glass.
U.S. Pat. No. 3,970,037 to Sopko discloses a coating composition vaporizer which may be employed to vaporize a vaporizable coating reactant for subsequent delivery through a nozzle or otherwise into contact with a substrate to be coated by a chemical vapor deposition technique. Solid reactants for use in the deposition of films by chemical vapor techniques have been vaporized in a variety of ways. Typically, a fixed bed of particulate solid coating reactant is maintained in a vaporization chamber such as shown in U.S. Pat. No. 2,780,553 to Pawlyk. A hot carrier gas is passed through the fixed bed of coating reactant to vaporize the reactant and to carry the vaporized reactant from the chamber in which the fixed bed of reactant resides to a substrate upon which a film is to be deposited. One of the problems associated with the use of a typical fixed bed coating reactant vaporizer is caused by the tendency for many pyrolytic coating reactants in particular to fuse into a sticky mass when heated so that the vaporizer becomes clogged with liquid coating reactant before the coating reactant can be effectively vaporized. Another problem associated with the use of fixed bed vaporizers has been preignition or the premature combustion or reaction of pyrolytic coating reactants when hot air is employed as a carrier gas.
In accordance with the teachings of both Sopko and Bloss et al, it has already been appreciated that the problems associated with premature combustion or reaction of pyrolytic coating reactant or the undesired fusion of such reactants may be minimized if coating reactants are successfully dispersed in a carrier gas prior to being heated to a sufficient temperature for vaporization of them. Sopko specifically teaches that a spray of mist, fog or smoke of coating reactant into a vaporization chamber is effective for accomplishing vaporization of coating reactants which are unstable to the extent that they are fused or preignited by premature combustion or reaction. Bloss et al specifically teach that it is desirable to disperse a particulate or powdered solid coating reactant prior to vaporization and teach the use of powdered solid coating reactants having particular thermodynamic properites which make them less susceptible to preignition or fusion. The dispersion techniques which have been employed in the past include, in addition to the spray procedure disclosed by Sopko, the distribution of a particulate or powdered solid coating reactant over a large supporting surface such as a tray or a screen.
The present invention contemplates a method for dispersing particulate solid coating reactants prior to their vaporization which are efficient dispersion techniques employing apparatus having essentially no moving parts which are subject to contact by the coating reactants and do not require trays or other devices which may be subject to clogging in the event of inadvertent fusion of coating reactants.